Three dimensional (3D) time-of-flight (TOF) cameras are active-type optical depth measurement imaging systems. In general, TOF cameras are based on the phase-measurement technique of emitted intensity-modulated light, which is reflected by the scene. The reflected light is imaged onto a sensor comprising a two-dimensional array of pixels. The photo-generated electrons are synchronously demodulated in the sensor, and based on the phase information, the distance for each pixel is deduced. A more detailed description is presented in U.S. Pat. No. 7,884,310, which is incorporated herein by this reference in its entirety.
A major problem of the state-of-the-art 3D TOF cameras is that cameras operating with the same modulation frequency interfere with each other. Different technologies are known to overcome this interference issue, however.
If more than one camera is operated, the cameras can be multiplexed in time (time-division multiplexing). In that case, the cameras a) need to be synchronized with one another and b) are slowed down in their individual frame rates.
Space-division multiplexing (SDMA) just separates the different cameras spatially, which is not an option for many applications.
Frequency multiplexing (FDMA) is another well-known method to avoid interference between cameras. Applying different modulation frequencies to the different cameras solves the problem of interference, but adds complexity in production and calibration and the cameras' resolution is different dependant on the applied modulation frequency.
The wavelength division multiplexing technique (WDMA) assigns a unique optical emission and detection wavelength to each camera. WDMA would restrict the number of cameras that can be used, however. It also increases challenges in manufacturing the different cameras.
Code division multiplexing (CDMA) is another common approach to avoid interference. By equally coding the emitted light and the receiver, the camera can unambiguously detect its own “light,” and interference is generally reduced. However, practical implementation so far have shown that the power budget of cameras applying a CDMA modulation technique is always worse compared to the pure sine modulation. The theoretical derivation of a TOF camera using pseudo-noise modulation as well as the practical results have been presented in B. Büttgen et al, “Pseudo-noise Optical Modulation for Real-Time 3-D Imaging With Minimum Interference”, IEEE transaction on circuits and systems, VOL. 55, NO. 6, July 2008.
All these possible modulation techniques and drawbacks are described in B. Büttgen, “Extending Time-of-Flight Optical 3D-Imaging to Extreme Operating Conditions”, Ph.D. thesis, University of Neuchatel, 2006.
More recently, a method to reduce camera interferences was described in U.S. Pat. No. 7,405,812. The method proposes to vary the frequency during the exposure (frequency hopping), add temporarily imperfections (waveform, periodicity), and include clock noise. All those propositions promise good multi-camera operability but require either highly complex clocking circuitry or risk unstable behavior.